1. Field of the Invention
This invention relates generally to the field of network circuitry. More particularly, the invention relates to a more versatile line interface for connecting a receiver to various transport mediums (e.g., data lines) hosting transport protocols that have different impedance requirements.
2. Description of Related Art
Various transport protocols are used to transport data across transport mediums. Transport protocols are defined by various industry specifications. For example, the T1 protocol (also called DS1) is specified by the American National Standards Institute (latest revision T1.403.00, 403.01, 403.02-1999). The E1 protocol (or E-1) is a European digital transmission format devised by the International Telecommunication Union (ITU-T) and is compliant with G703 (latest revision October, 1998). Similarly, the J1 protocol, used in Japan, is specified by the Telecommunication Technology Committee (latest revision JT-G703, April, 1989).
When a connection is made to a transport medium hosting a particular transport protocol, the line interface, or connector, must have a substantially similar impedance to that specified by the transport protocol, otherwise electrical reflection will occur causing interference, as is well known.
FIG. 1 shows a prior art line interface for connecting a receiver to a transport medium (line 110). The line 110 is connected to a tip input 140 and a ring input 142 of the receiver portion of transceiver 150 via a transformer 120. A resistor 130 is used to provide an impedance for matching the impedance of line 110. For example, a T1 line according to the ANSI T1 specification has a nominal terminating impedance at the interface of 100 ohms. The T1 specification also specifies a return loss with respect to 100 ohms over the frequency band from 100 kHz to 1 Mhz of at least 26 dB.
The return loss is determined by the equation:
20 log10 [(ZT+ZL)/Abs(ZT−ZL)], where ZT is the impedance of the line interface, ZL is the impedance of the line, and Abs is the absolute value function. It is clear that the closer ZT is to ZL, the higher the return loss. If ZT is not close enough to ZL, the return loss requirement of the T1 specification will not be met. The term “substantially match” used herein denotes meeting the specification of the transport protocol.
In this prior art line interface, if the transceiver is subsequently connected to a transport medium that has a different impedance requirement, then the external resistor is physically replaced with a corresponding resistor. For example, a J1 line requires a nominal termination impedance of 110 ohms; an E1 line (coax line) requires a nominal termination impedance of 75 ohms; and an E1 line (twisted pair line) requires a nominal termination impedance of 120 ohms. In the prior art line interface, the external resistor would be switched to match the nominal termination impedance of the new transport medium and transport protocol.